1. Field of the Invention
This invention relates to pneumatic radial tires for construction vehicles, which are mounted on large size dump trucks used on large scale civil engineering sites and ore mining sites and used on a rough ground surface under severer conditions at a heavy load. More particularly it relates to an improvement in bead durability of such a tire.
2. Related Art Statement
When the pneumatic radial tire for construction vehicles is compared with pneumatic radial tires for truck and bus under the same heavy load, it is used under conditions that the speed range to be used is low and the load is high at the same tire size (under the same internal pressure) or the internal pressure is low (under the same load).
In the pneumatic radial tire for construction vehicles, the flexible amount ranging from the sidewall portion to the bead portion is large as compared with the other heavy duty tires and also the trouble in the bead portion is frequently caused due to the application of a large load during the running on considerably uneven rough roads, so that the improvement of the bead portion durability is unavoidable in order to enhance the reliability of the pneumatic radial tire for construction vehicles in use.
Heretofore, the tire of this type is provided with a carcass comprised of rubberized steel cord ply(s) toroidally extending between a pair of bead cores and turned around the bead core from inside of the tire toward outside thereof as a body reinforcement, and at least two chafers each comprised of rubberized nylon cord fabric or steel cord fabric, cords of which chafers being crossed with each other, and arranged along the carcass ply turnup as a bead portion reinforcement in the bead portion having a skeleton structure of bead core and carcass ply turnup region surrounding the bead core.
The use of the chafer enhances the rigidity of the bead portion, but when it is repeatedly subjected to compression deformation during running of the tire, which is particularly conspicuous in this type of the tire, the turnup portion of the carcass ply and the chafer portion frequently and easily cause the fatigue fracture and also the rigidity required in the bead portion can not sufficiently be ensured and there is a risk of developing from rubber cracking along the turnup portion of the carcass ply comprised of steel cords and at the end of the chafer toward the separation failure.
On the other hand, bending rigidity can particularly be enhanced by increasing the thickness of the stiffener rubber placed between the carcass ply and the turnup portion thereof on the bead core. But, when it is repeatedly subjected to compression deformation, there is a fear of breaking the turnup portion of the carcass ply and hence there is caused a separation failure near to the carcass ply.